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Comparative cleaning tests with modified protein and starch residues
Journal of Food Engineering 178 (2016) 145e150
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Comparative cleaning tests with modified protein and starch residues C. Otto a, S. Zahn a, M. Hauschild a, F. Babick b, H. Rohm a, * a b
€t Dresden, 01062, Dresden, Germany Chair of Food Engineering, Technische Universita €t Dresden, 01062, Dresden, Germany Institute of Process Engineering and Environmental Technology, Technische Universita
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 October 2015 Received in revised form 7 January 2016 Accepted 18 January 2016 Available online 21 January 2016
The cleanability of food contact surfaces is determined by surface characteristics and the properties of the soiling material. Effects of selected physicochemical parameters of biopolymer soils on the removal from stainless steel were studied using a laboratory flow cell system with continuous removal measurement and, additionally, gravimetric detection. The results show the potential for improving cleaning processes on the basis of structural properties of the biopolymers. A higher cleaning efficiency was achieved when surface energy and cationic charge of the polymers was lower, indicating that adhesive interactions (van der Waals forces of attraction and electrostatic forces) between surface and polymer play a dominant role. The effects of pre-heating treatments of the soils, inducing molecular changes, on removal behavior were however not significant. The results indicate that a holistic view of product and process design is useful to develop efficient cleaning protocols. © 2016 Published by Elsevier Ltd.
Keywords: Cleaning Fouling Flow cell Stainless steel Protein Starch
1. Introduction Cleaning is essential to ensure hygienic food production and process performance. To improve efficiency of the cleaning of either heated or unheated surfaces, the knowledge of the specific soil properties that affect the performance of cleaning processes is essential. The understanding how organic soils respond to cleaning assists to finding decisions concerning cleaning fluid and method (Fryer and Asteriadou, 2009; Wilson, 2005). To execute efficient cleaning, adhesive and cohesive bindings of a soil onto contact surfaces must be overcome with minimal energy. The mechanisms of particle or polymer removal are influenced by the physicochemical properties of the soiling material and can be explained by thermodynamic and electrostatic fundamentals (Fourche, 1995; Michalski et al., 1997; Visser, 1995). According to the DLVO theory, total interaction energy comprises Lifshitzevan der Waals attractive forces, and attractive or repulsive electrostatic forces. Van der Waals interactions itself depend mainly on the Lifshitzevan der Waals (LW) surface free energy of the soil, and the electrostatic interaction is controlled by its electrokinetic potential. In contrast to electrostatic forces, van der Waals contributions are
€t * Corresponding author. Chair of Food Engineering, Technische Universita Dresden, Bergstrasse 120, 01062 Dresden, Germany. E-mail address: [email protected] (H. Rohm). http://dx.doi.org/10.1016/j.jfoodeng.2016.01.015 0260-8774/© 2016 Published by Elsevier Ltd.
insensitive to pH variation and electrolyte concentration in aqueous systems (Al-Amoudi and Lovitt, 2007; Zhao and Liu, 2006). Contact area effects, depending on soil particle roughness and morphology, play an additional role in polymer/surface interactions (Bobe et al., 2007; Liu et al., 2006). Several physical and chemical processes that alter the structure of organic soils may influence adhesion to contact surfaces. For example, acidification leads to systems with a higher cationic charge and hydrophobicity, which increases attractive forces so that removal becomes more difficult (Lopez et al., 2010; Stanga, 2010). Mauermann et al. (2009) reported a decrease of cohesive interactions within the soil and between soil and surface when surface and protein or starch-based soils have the same electrical charge. Itoh et al. (1995) explained electrostatic interactions between b-lactoglobulin and stainless steel by modified amino or carboxyl groups. For several starch types, Prochaska et al. (2007) showed that polar carboxyl group substitution resulted in enhanced surface activity and was accompanied by a viscosity decrease, especially at pH < 4.0. Drying removes water from a substrate and leads to changes such as volume reduction, solids redistribution and polymerization. Closer packing increases cohesive forces in films and makes removing and dispersing soils more difficult (Jonhed et al., 2008; Stanga, 2010). Additionally, LW surface free energy gLW of the soil will change. In context with tomato paste removal from stainless steel, Liu et al. (2006) reported on an adhesive strength of gLW ¼ 29.2 mJ/m2 for the unbaked system, and of
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gLW ¼ 30.6 mJ/m2 for tomato paste baked onto the steel. Detry et al. (2011) revealed an influence of the drying rate on starch granule adherence, which was attributed to physico-chemical wetting mechanisms. Exposure to more than 60 C also causes structural changes. It was reported that, when globular proteins undergo shape changes through denaturation, the exposure of hydrophobic sites leads to insolubilization. Surface hydrophobicity affects shape and compactness of adhering aggregates at soiling and their interfacial behavior during cleaning (Sava et al., 2005; Rabe et al., 2011). Goode et al. (2013) observed a significant temperatureinduced increase in adhesion forces between whey protein and stainless steel. When starch is gelatinized in water, colloidal solutions of high viscosity are obtained, in which the internal structures of the granules undergo considerable changes (Ratnayake and Jackson, 2006). The need to study and optimize cleaning has contributed to the development of specialized research techniques including flow cells, micromanipulation devices, or fluid dynamic gauging (Gordon et al., 2014). To evaluate soil adherence to surfaces, the residual soil mass after applying washing or cleaning protocols on solid surfaces can be determined (Handojo et al., 2009; Michalski et al., 1997). Flow cells were applied for comparative investigation of the cleaning behavior of different soils (Detry et al., 2009; Otto et al., 2014). Sensitive and rapid spectroscopic techniques may be used to continuously assess the amount of polymers removed from stainless steel, or in cleaning solutions (Boyd et al., 2001; Fickak et al., 2011). Several studies concerning the removal of several types of food soils were performed to investigate effects of surface modification treatments at different scale level, to determine properties of cleaning agents, or to develop or to monitor cleaning techniques (e.g., Boxler et al., 2013; Jeurnink and Brinkman, 1994; Saikhwan €ußer et al., 2012). The cleanet al., 2006; Otto et al., 2011; Wallha ing mechanism is also influenced by type and properties of the residues which are to be removed. A general relationship between soil characteristics and cleaning energy is given by Fryer and Asteriadou (2009) who developed the cleaning map to classify cleaning problems. However, little work has been done to extend the classification matrix to cohesive solid soil residues. This article demonstrates effects of selected modification procedures of starch and protein on the cleaning result. The parameters analyzed were the electrokinetic potential, surface free energy and conformational changes of a range of polymer residues treated by various process conditions such as acidification, drying and heating. 2. Materials and methods 2.1. Materials The test specimens had a surface roughness 0.8 mm, which is generally recommended for being used in the food industry (Gerhards and Schmid, 2013). The coupons from electropolished stainless steel 316L (40 20 1 mm3) were washed in 1% NaOH under sonication (15 min, 45 kHz, 160 W), rinsed in deionized water and dried on filter paper. Other surface properties of the steel coupons are: surface free energy, 38.3 mJ/m2; isoelectric point, 4e5 (Mauermann et al., 2009). Milei 80 whey protein (Milei GmbH, Leutkirchen, Germany), soy protein (Vegacon 90, Nutrition-Factory Alphacaps GmbH, Augustdorf, Germany), native waxy corn (WC) starch (Maisita 21.000, €rke GmbH, Aschach, Austria) and oxidized potato (OP) Agrana Sta starch (Aganadyn 20.050) were selected as model polymers. Moisture was determined by drying at 105 C to constancy (ISO, 1996), fat content was determined by the Soxhlet method (ISO, 1994), and protein by the Kjeldahl method (ISO, 2005) using
conversion factors of 6.25 for whey protein and 5.71 for soy protein. Ash was determined by incineration (ISO, 2007), and carbohydrate content was calculated as remaining difference to 100%. The amylose content of starch was analyzed using the concanavalin-A binding method (Yun and Matheson, 1990) with a commercial test kit (Megazyme Ltd., Bray, Ireland) (Table 1). 2.2. Soiling material preparation and modification Based on suggestions from literature (Boyd et al., 2001; Detry et al., 2011), each powder was suspended in deionized water at a concentration of 1% (w/v). After stirring for 12 h at room temperature, 100 mL liquid soil (thus containing 1 mg polymer) was pipetted onto a 2 cm2 area of the steel coupons and dried at 55 C for 1 h. In addition to this standard soiling procedure, modified polymer residues were prepared by changing pH and/or drying temperature, or by pre-heating the polymer suspensions to mimic processing conditions. pH adjustment was done with 0.5% HCl (pH 3), or with 0.5% NaOH (pH 9). To simulate thermal exposure, the soil films on the coupons were dried at 90 C for 1 h. Heat-induced modification of the polymers prior to soiling was achieved by heating the suspensions in a water bath to 82 C for 2 min. 2.3. Characterization of test soils 2.3.1. Electric properties Streaming potential and isoelectric point (IEP) of acidified or alkalized polymer suspensions were determined using a PCD 03 pH particle charge detector (Mütek, Herrsching, Germany; Mohammed et al., 2000). After filling 20 mL 1% (w/v) polymer suspension into the teflon cell, the streaming potential was induced by a plunger oscillating in the sample cell. Measurement temperature was 21 C. pH was adjusted by 0.1 mol/L HCL or 0.1 mol/L NaOH in 0.05 mL/min steps with an automatic titration device (Metrohm AG, 702 SM). The potentials (mV) were recorded in duplicate together with suspension pH by PCD-Titration software 1.6. In the titration curve the polymer IEP refers to pH at a streaming potential of zero. 2.3.2. Thermodynamic properties Contact angles of polymer films were determined by a modified Wilhelmy plate method using a digital DCAT11 tensiometer (Dataphysics, Filderstadt, Germany; Stiller et al., 2004). The stainless steel coupon was coated on both sides with thin coherent films by sequential pipetting of 100 mL polymer suspension on each side and subsequent drying at 55 C or 90 C for 1 h. The plate was attached to the instrument's microbalance (accuracy: 0.01 mg) and immersed into a-bromonaphthalene (a-BN) at 2 mm/min. The contact angle q ( ) was calculated from the wetting force F (mN) by cosq ¼ F/gL where g is the surface tension of a-BN (44.4 mJ/m2) and L (m) is plate perimeter. From the average contact angle cosine of the apolar a-BN, the Lifshitzevan der Waals component of the surface free energy gLW (mJ/m2) of dry soil films was calculated using gLW ¼ 11.1 (1 þ cosq)2 (Zhao et al., 2004). Analysis was done in 10 replicates. 2.3.3. Thermal properties Thermal properties of the biopolymer soil residues were analyzed in duplicate by differential scanning calorimetry (DSC) from 20 to 110 C at a heating rate of 10 K/min using a DSC Q200 with hermetic aluminum pans (TA Instruments, Eschborn, Germany). The soils were detached from coupons by placing them in 10 mL deionized water in a T480/H-2 ultrasonic bath (Elma GmbH, Singen, Germany) for 1 h and subsequently concentrated with an
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Table 1 Chemical composition of soil raw materials. Proteinsa
Parameter
Moisture (%) Protein (%) Fat (%) Total carbohydrates (%)b Amylose (%) Ash (%) a b
Starchesa
Whey
Soy
Waxy corn
Oxidized potato
8.3 ± 0.2 76.1 ± 0.1 5.2 ± 0.2 6.2 e 4.2 ± 0.2
7.1 ± 0.1 88.2 ± 0.3 0.5 ± 0.4 0.4 e 3.8 ± 0.1
11.7 ± 0.3 0.2 ± 0.1 0.3 ± 0.1 87.0 3.3 ± 0.2 0.8 ± 0.1
14.3 ± 0.2 0.3 ± 0.1 0.4 ± 0.1 84.1 29.7 ± 0.5 0.9 ± 0.1
Arithmetic mean ± standard deviation from (n ¼ 3) independent replicates. Total carbohydrates is [100 (moisture þ protein þ fat þ ash)].
Alpha 1e2 freeze dryer (Martin Christ Gefriertrocknungsanlagen GmbH, Osterode, Germany). 2.5 mg duplicates of the lyophilized samples were weighed into the pans, and 10 mL of deionized water was added. An empty pan was used as reference. Data analysis was done with Universal Analysis 2000 V4.4 software. 2.4. Cleaning experiments Cleaning experiments were carried out using a continuous flow cell system in the close circuit measuring arrangement (Otto et al., 2014) in four replicate measurements. Fig. 1 shows the main components of this device: a 50 mL reservoir with a magnetic stirrer for the cleaning fluid, the flow cell with a volume of 3 mL (width 6 cm, length 10 cm), and a peristaltic pump. After fixing the soiled stainless steel coupon in the flow cell and closing by four wing nuts, cleaning tests were performed with water (for protein removal) or Lugol's solution (0.007% J2, 0.014% KJ, pH 6.1; for starch removal) as cleaning liquids at a flow rate of 54 mL/min for 2 min periods. This flow rate corresponds to a Reynolds number of approx. 40 (Detry et al., 2009; Otto et al., 2014). To simulate prerinsing which is normally included in every cleaning procedure, cleaning liquid temperature was 21 C. To monitor cleaning progress, polymer concentration in the cleaning liquid was continuously measured using a Lambda 2S UV/ VIS spectrophotometer (see Fig. 1; Perkin Elmer, Rodgau, Germany). The concentration was directly proportional to absorbance up to 300 mg/mL protein in aqueous solution at 230 nm, and that of starch in Lugol's solution at 600 nm (Otto et al., 2014). Actual soil removal was calculated from the cleaning system fluid volume multiplied by protein or starch concentration in the fluid, and cleaning efficiency CE is further expressed as the ratio of soil removed at time t to the initial soil on the coupon (i.e., 1 mg). Based on the results of a previously published study (Otto et al., 2014), the removal after 2 min of continuous cleaning was defined as final cleaning efficiency (CEt¼2 min). In addition to spectroscopic determination, CEt¼2 min was
Fig. 1. Schematic flow cell device for cleaning tests.
gravimetrically determined for selected samples using the microbalance of the DCAT 11 tensiometer. After cleaning of the soiled steel coupons in the flow cell for 2 min, the coupons were dried for 1 h at 55 C and weighed with the built-in balance. Gravimetric cleaning efficiency GCEt¼2min was calculated by GCEt¼2min ¼ 100 [1 (mr mu)/(mi mu)] where mr, mi and mu are the mass of the remaining dried soil with coupon after cleaning, the initial mass of dried soil before cleaning, and the unsoiled dry coupon, respectively. 3. Results and discussion 3.1. Effect of acid or alkaline treatment Fig. 2 presents the streaming potential of the polymers at 21 C, which generally increased from pH 9 to pH 3. Whey protein and soy protein showed IEPs of approx. 4.6 and 5.1, respectively, which is in agreement with literature (Itoh et al., 1995). Mohammed et al. (2000) also demonstrated that a much higher temperature treatment intensity than applied in this study during soil drying is necessary to affect the IEP of dairy proteins to a significant extent. At acidic conditions (pH ~3), both starches exhibited a streaming potential close to zero. For OP starch, the potential remained negative over the investigated pH range because of the negative charges of carboxyl groups induced by oxidation (Kuakpetoon and Wang, 2008; Piorkowski and McClements, 2014). WC starch showed an IEP of ~5.5, indicating the contribution of the charged groups on granules surface. Fig. 3 shows removal profiles (i.e., cleaning characteristics measured by the continuous spectroscopic method) for whey
Fig. 2. pH dependency of the streaming potential of polymer soil solutions. Circles, whey protein; squares, soy protein; triangles, oxidized potato starch; diamonds, waxy corn starch. Data are arithmetic mean ± half deviation range from duplicate measurements.
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Fig. 3. pH dependency of the continuous spectroscopic cleaning profiles of whey soil solutions dried at 55 C. Data are arithmetic mean ± standard deviation from (n ¼ 4) replicate measurements. Gravimetric cleaning efficiency after 2 min is given for comparison.
suspensions of different pH, which were dried onto the steel coupons at 55 C. These profiles show that the cleaning efficiency calculated from the concentration in the soiling solution approaches a plateau and, after 2 min, gives a characteristic value to compare the effects of modification treatments. Our previous study (Otto et al., 2014) revealed that increasing the flow rate to 74 mL/ min (Re ~ 56) increased the initial cleaning rate but not the final CEt¼2min value. The whey soil pH shows a pronounced effect on the cleaning efficiency. Furthermore, CEt¼2min detected by the spectroscopic method is in good agreement with the gravimetric reference value GCEt¼2min. The determination of all other CEt¼2min profiles further referred to in this study was based on the recording of the respective cleaning profiles (data not shown). The appearance of these profiles was similar, meaning that soil removal started with a high rate and then approached the CEt¼2min value asymptotically. For a total of eight soils, the efficiency of cleaning for a 2 min time span was measured by both methods (Table 2). In only one of the comparisons (two-sample two-tailed t-test), a significant (P < 0.05) difference between the results was observed. The obtained values are highly correlated (R ¼ 0.98), and the linear regression function CEt¼2min (%) ¼ 0.23% þ 1.01 GCEt¼2min (%) clearly proves the reliability of the spectroscopic method. In addition, the average coefficients of variation of 11.3% (gravimetric) and 4.8% (spectroscopic determination) demonstrate the accuracy of the continuous flow cell method. Fig. 4 presents CEt¼2 min for all polymers at pH 3, 6 and 9 as a function of their streaming potential; this CEt¼2 min shows a general
Fig. 4. Effects of streaming potential and pH on the cleaning efficiency CEt¼2min of polymer suspensions dried at 55 . Circles, whey protein; squares, soy protein; triangles, oxidized potato starch; diamonds, waxy corn starch. Cleaning data are arithmetic mean ± standard deviation from (n ¼ 4) replicate measurements.
trend towards higher values when soil solution pH was higher. Because the IEP of stainless steel is reported as being in the acidic pH region (Mauermann et al., 2009), both the starches and the steel are negatively charged at alkaline pH. This suggests that electrostatic repulsive forces dominate and bonding forces are low. In the acidic milieu, a reduced charge of the starch particles decreases repulsive forces and causes lower cleaning efficiency. The proteins also exhibit a higher cleaning efficiency at higher pH; the highest removal was however observed when the soil solution was neutral (pH 6). Because repulsive forces diminish at this pH (which also implies lower adhesive removal) it is suggested that a cohesive removal behavior or other types of interaction forces (e.g., acidebase interactions) dominate. Generally, the protein film formation in the acidic range can be regarded as a very complex process. Gennadios et al. (1993) reported that film formation was inhibited by a poor protein dispersion around the IEP region whereas, at a pH away from the IEP, proteins may denature, unfold, or solubilize, thus exposing sulfhydryl and hydrophobic groups (Castner and Ratner, 2002). Such groups associate upon drying to form disulfide and hydrophobic bonding forces (Gennadios et al., 1993). It is common for all polymers that a higher electrokinetic potential in the acidic range enhances polymer removal and leads to a decrease of the strength of repulsive electrostatic forces between polymer and surface. Takahashi and Fukuzaki (2008) and Mauermann et al. (2012) also suggested that a better cleanability of metal oxide surfaces was probably caused by the small binding strengths of biopolymers onto negatively charged metal surfaces,
Table 2 Comparison of cleaning efficiency determination. Sample
Cleaning efficiency after 2 min (%)a Spectroscopic determination
Whey protein pH 9 Whey protein pH 6 Whey protein pH 3 Soy protein pH 6 Oxidized potato starch pH 9 Oxidized potato starch pH 6 Oxidized potato starch pH 3 Waxy corn starch pH 6
a
78.9 99.7a 66.0a 98.8a 52.8a 38.9a 26.0a 4.1a
± ± ± ± ± ± ± ±
2.40 3.29 0.91 2.91 3.75 3.88 1.49 1.47
Gravimetric determination 86.4b 98.1a 62.8a 94.0a 48.7a 42.2a 23.8a 4.5a
± ± ± ± ± ± ± ±
4.30 1.93 3.88 2.88 4.97 5.41 4.72 1.43
a Arithmetic mean ± standard deviation from (n ¼ 4) independent replicates. Mean values in a row with different superscripts differ significantly (P < 0.05; two-sample two-tailed t-test).
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and increases OP starch removal, with no apparent correlation to polymer LW surface free energy (within polymer type). However, concerning the entire polymer data set it appears that higher gLW and thus higher adhesion forces contribute to a higher adherence of polymer residues (the linear correlation coefficient is R ¼ 0.88), and their removal becomes more difficult. Differences in the relation between surface free energy and protein removal may result from the cohesive removal behavior or specific forces (electrostatic, acidebase) that counteract attractive van der Waals forces (Mauermann et al., 2012). Jonhed et al. (2008) reported that starch films formed at 95 C for 30 min are flexible, and their cohesive and adhesive strength decrease with increasing moisture content. Bobe et al. (2007) observed a similar trend between cleaning efficiency and surface energy by using one particular soil system and by varying the soiled surface. Additionally surface forces affect the polymer solubility (Van Oss, 1993), and it was also found that an intensive heat-induced modification treatment reduces protein solubility and increases starch solubility (Sava et al., 2005).
Fig. 5. Effect of Lifshitzevan der Waals (LW) surface free energy of the polymer on cleaning efficiency CEt¼2min. Circles, whey protein; squares, soy protein; triangles, oxidized potato starch; diamonds, waxy corn starch. Data are from cleaning of pH 6 soils and given as arithmetic mean ± standard deviation from (n ¼ 4) replicate measurements.
3.3. Effect of preheating treatment Table 3 presents the thermal DSC transition parameters and cleaning efficiency at 2 min of soil residues made from different preheated soil solutions. The pH of the solutions was only marginally affected by the heating step. For whey protein, OP and WC starch it is evident that the preheating treatment decreases the transition enthalpy but does not influence cleaning efficiency (P > 0.05). The ratio of the transition enthalpies of untreated to preheated samples reflects the degree of structural modifications that is caused by heat-induced denaturation or gelatinization. No transition peak and no unfolding temperature were detected in the soy protein thermograms of unheated and preheated residues, indicating a severe thermal treatment during the manufacture of the powder. The transition temperatures for whey protein, OP and WC starch reflect their conformational stability and are in agreement with literature (Ratnayake et al., 2009; Santos et al., 2006). It is especially the gelatinization temperature that can be critical for a specific cleaning action because intragranular binding forces of starches that are weakened facilitate the solubilization of granule constituents (Lopez et al., 2010). Our cleaning results indicate that the conformational changes of the polymers on the solid surface largely depend on polymer conformation in suspension and give only a minor contribution to their cleaning behavior. Furthermore, other cleaning studies in three-component systems with polymer, cleaning fluid and contacting stainless steel surface with a defined roughness from 0.2 to 1.8 mm reported no significant effects for food polymers (Gerhards and Schmid, 2013; Mauermann et al., 2012; Saikhwan et al., 2006).
and due because of their negative charge under alkaline conditions. €rnstro € m (2003) reported that the binding power of Jonhed and Ja cationic starch is higher than that of native starch because the ionic interactions are stronger than simple hydrogen bonds. In a fundamental study of solid particle detachment from membrane surfaces, Elzo et al. (1996) also observed that decreasing the pH in acid range increased the adhesive force between solid particles. 3.2. Effect of drying treatment Fig. 5 shows the effect of the temperature of drying soil solutions of pH 6 on LW energy and cleaning efficiency. The temperature increase from 55 to 90 C significantly (two-sample two-tailed ttest; P < 0.05) increases LW surface free energy of the globular proteins because of the heat-induced exposure of hydrophobic sites (Sava et al., 2005). LW surface free energy of starches was not affected, indicating a similar hydrophobic character of the materials after drying (Jonhed et al., 2008; Liu et al., 2006). The largest differences in surface free energy were observed between polymer types. For example, WC starch showed the highest gLW (approx. 40 mJ/m2). The energy values are in agreement with literature and allow a satisfactory comparison of the polymer surface properties (Cyras et al., 2008; Liu et al., 2006; Zhao and Liu, 2006). For example, Lawton (1995) used Wu's approach (Mauermann et al., 2009) to calculate the dispersive, non-polar surface energy of different extruded corn starch films. This measure with energy values in the range from 35 to 42 mJ/m2 roughly corresponds to the non-polar LW component of surface free energy that was measured in this study. A higher drying temperature of 90 C decreases protein removal
4. Conclusion The flow cell system was successfully used for the comparative
Table 3 Effect of soil solution preheating on thermal properties and cleaning efficiency CEt¼2min values of polymer residues. Polymer source
Unheated, 21 C pH
Whey protein Soy protein OP starch WC starch a
6.4 6.4 6.5 5.9
Pre-heated, 82 C, 2 min
Peak temperature Tp ( C) 76.1 ± 0.1 n.d. 63.6 ± 0.2 73.3 ± 0.3
a
1 a
Enthalpy DH (J g 6.9 ± 0.1 0 13.3 ± 0.1 12.9 ± 0.2
)
CEt¼2min (%) a
99.7 98.8a 38.9a 4.1a
± ± ± ±
3.2 2.9 3.8 1.4
b
pH
Peak temperature Tp ( C)
6.3 6.3 6.6 6.1
n.d. n.d. 64.2 ± 0.2 n.d.
a
Enthalpy DH (J g1) 0 0 1.2 ± 0.1 0
a
CEt¼2min (%)b 98.7a 99.8a 44.4a 6.2a
± ± ± ±
1.1 4.9 2.5 1.5
Arithmetic mean ± half deviation range from duplicate measurements. n.d., not detectable. Arithmetic mean ± standard deviation from (n ¼ 4) measurements. Mean values in a row with different superscripts differ significantly (P < 0.05; two-sample two-tailed ttest). b
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investigation of the cleaning behavior of modified protein and starch films from stainless steel plates. The modification treatments of the protein and starch soil solutions, i.e., changing drying temperature and soil solution pH, affected physicochemical soil parameters and controlled the cleaning efficiency. For all soils, the time course of the removal followed a sigmoid function. Higher cleaning efficiency values were observed in case of lower energetic or lower cationic charged biopolymers, which can be attributed to Lifshitzevan der Waals or electrostatic interactions. The effects of the thermally induced structural changes on biopolymer removal were however not significant. The results indicate that the knowledge of soil biopolymer characteristics induced by processing conditions is useful to create tailor-made cleaning protocols. Furthermore, investigations into the physic-chemical real time characterization of soils in processing plants may be a promising approach to reduce cleaning efforts. References Al-Amoudi, A., Lovitt, R.W., 2007. Fouling strategies and the cleaning system of NF membranes and factors affecting cleaning efficiency. J. Membr. Sci. 303, 4e28. Bobe, U., Hofmann, J., Sommer, K., Beck, U., Reiners, G., 2007. Adhesion e where cleaning starts. Trends Food Sci. Technol. 18, S36eS39. Boyd, R.D., Verran, J., Hall, K.E., Underhill, C., Hibbert, S., West, R., 2001. The cleanability of stainless steel as determined by X-ray photoelectron spectroscopy. Appl. Surf. Sci. 172, 135e143. Boxler, C., Augustin, W., Scholl, S., 2013. Cleaning of whey protein and milk salts soiled on DLC coated surfaces at high-temperature. J. Food Eng. 114, 29e38. Castner, D.G., Ratner, B.D., 2002. Biomedical surface science: foundations to frontiers. Surf. Sci. 500, 28e60. zquez, A., 2008. Physical and meCyras, V.P., Manfredi, L.B., Ton-That, M.-T., Va chanical properties of thermoplastic starch/montmorillonite nanocomposite films. Carbohydr. Polym. 73, 55e63. Detry, J.G., Deroanne, C., Sindic, M., 2009. Hydrodynamic systems for assessing surface fouling, soil adherence and cleaning in laboratory installations. Biotechnol. Agrono., Soc. Environ. 13, 427e439. Detry, J.G., Sindic, M., Servais, M.J., Adriaensen, Y., Derclaye, S., Deroanne, C., Rouxhet, P.G., 2011. Physico-chemical mechanisms governing the adherence of starch granules on materials with different hydrophobicities. J. Colloid Interface Sci. 355, 210e221. Elzo, D., Schmitz, P., Houi, D., Joscelyne, S., 1996. Measurement of particle/membrane interactions by a hydrodynamic method. J. Membr. Sci. 109, 43e53. Fickak, A., Al-Raisi, A., Chen, X.D., 2011. Effect of whey protein concentration on the fouling and cleaning of a heat transfer surface. J. Food Eng. 104, 323e331. Fourche, G., 1995. An overview of the basic aspects of polymer adhesion. Part I Fundam. Polym. Eng. Sci. 35, 957e967. Fryer, P.J., Asteriadou, K., 2009. A prototype cleaning map: a classification of industrial cleaning processes. Trends Food Sci. Technol. 20, 255e262. Gennadios, A., Brandenburg, A.H., Weller, C.L., Testin, R.F., 1993. Effect if pH on properties of wheat gluten and soy protein isolate films. J. Agric. Food Chem. 41, 1835e1839. Gerhards, C., Schmid, A., 2013. Assessing the cleanability of stainless steel surfaces e development of a testing method for starch and protein based soils. J. Hyg. Eng. Des. 3, 9e14. Goode, K.R., Bowen, J., Akhtar, N., Robbins, P.T., Fryer, P.J., 2013. The effect of temperature on adhesion forces between surfaces and model foods containing whey protein and sugar. J. Food Eng. 118, 371e379. € ler, M., Fo € ste, H., Helbig, M., Augustin, W., Chew, Y.M.J., Scholl, S., Gordon, P.W., Scho Majschak, J.-P., Wilson, D.J., 2014. A comparison of local phosphorescence detection and fluid dynamic gauging methods for studying the removal of cohesive fouling layers: effect of layer roughness. Food Bioprod. Proc. 92, 46e53. Handojo, A., Zhai, Y., Frankel, G., Pascall, M.A., 2009. Measurement of adhesion strengths between various milk products on glass surfaces using contact angle measurement and atomic force microscopy. J. Food Eng. 92, 305e311. ISO, 1994. Starches, Native or Modified e Determination of the Total Fat Content. Standard 3947. International Organization of Standardization, Geneva. ISO, 1996. Starch e Determination of Moisture Content e Oven Drying Method. Standard 1666. International Organization of Standardization, Geneva. ISO, 2005. Animal Feeding Stuffs e Determination of Nitrogen Content and Calculation of Crude Protein Content e Part 1: Kjeldahl Method. Standard 5983e1. International Organization of Standardization, Geneva. ISO, 2007. Cereals, Pulses and By-products e Determination of Ash Yield by Incineration. Standard 2171. International Organization of Standardization, Geneva.
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Liu, W., Fryer, P.J., Zhang, Z., Zhao, Q., Liu, Y., 2006. Identification of cohesive and adhesive effects in the cleaning of food fouling deposits. Inn. Food Sci. Emerg. Technol. 7, 263e269. Lopez, O.V., Zaritzky, N.E., García, M.A., 2010. Physicochemical characterization of chemically modified corn starches related to rheological behavior, retrogradation and film forming capacity. J. Food Eng. 100, 160e168. Mauermann, M., Eschenhagen, U., Bley, T., Majschak, J.-P., 2009. Surface modifications e application potential for the reduction of cleaning costs in the food processing industry. Trends Food Sci. Technol. 20, S9eS15. Mauermann, M., Bellmann, C., Calvimontes, A., Caspari, A., Bley, T., Majschak, J.-P., €chen in der Lebensmittelindustrie durch Flüs2012. Reinigbarkeit von Oberfla sigkeitsstrahlen. Chem. Ing. Tech. 84, 1568e1574. Michalski, M.-C., Desobry, S., Hardy, J., 1997. Food materials adhesion: a review. Crit. Rev. Food Sci. Nutr. 37, 591e619. Mohammed, Z.H., Hill, S.E., Mitchell, J.R., 2000. Covalent crosslinking in heated protein systems. J. Food Sci. 65, 221e226. Otto, C., Zahn, S., Rost, F., Zahn, P., Jaros, D., Rohm, H., 2011. Physical methods for cleaning and disinfections of surfaces. Food Eng. Rev. 3, 171e188. Otto, C., Zahn, S., Plenker, J., Rohm, H., 2014. Application of a flow cell for the comparative investigation of the cleaning behavior of starch and protein. J. Food Eng. 131, 1e6. Prochaska, K., Ke˛ dziora, P., Thanh, J.L., Lewandowicz, G., 2007. Surface activity of commercial food grade modified starches. Coll. Surf. B Biointerfaces 60, 187e194. Piorkowski, D.T., McClements, D.J., 2014. Beverage emulsions: recent developments in formulation, production, and applications. Food Hydrocoll. 42, 5e41. Ratnayake, W.S., Otani, C., Jackson, D.S., 2009. DSC enthalpic transitions during starch gelatinisation in excess water, dilute sodium chloride and dilute sucrose solutions. J. Sci. Food Agric. 89, 2156e2164. Ratnayake, W.S., Jackson, D.S., 2006. Gelatinization and solubility of corn starch during heating in excess water: new insights. J. Agric. Food Chem. 3712e3716. Rabe, M., Verdes, D., Seeger, S., 2011. Understanding protein adsorption phenomena at solid surfaces. Adv. Colloid Interface Sci. 162, 87e106. Saikhwan, P., Geddert, T., Augustin, W., Scholl, S., Paterson, W.R., Wilson, D.I., 2006. Effect of surface treatment on cleaning of a model food soil. Surf. Coat. Technol. 201, 943e951. n, K., Paulsson, M., Tra €gårdh, C., 2006. Effect of surSantos, O., Nylander, T., Schille face and bulk solution properties on the adsorption of whey protein onto steel surfaces at high temperature. J. Food Eng. 73, 174e189. Sava, N., van der Plancken, I., Claeys, W., Hendrickx, M., 2005. The kinetics of heatinduced structural changes of b-lactoglobulin. J. Dairy Sci. 88, 1646e1653. Stanga, M., 2010. Sanitation: Cleaning and Disinfection in the Food Industry. WileyVCH, Weinheim. Stiller, S., Gers-Barlag, H., Lergenmueller, M., Pflücker, F., Schulz, J., Wittern, K.P., Daniels, R., 2004. Investigation of the stability in emulsions stabilized with different surface modified titanium dioxides. Coll. Surf. A Physicochem. Eng. Asp. 232, 261e267. Takahashi, K., Fukuzaki, S., 2008. Cleanability of titanium and stainless steel particles in relation to surface charge aspects. Biocontrol Sci. 13, 9e16. Van Oss, C.J., 1993. Acid-base interfacial interactions in aqueous media. Coll. Surf. A 78, 1e49. Visser, J., 1995. Particle adhesion and removal: a review. Part. Sci. Technol. 13, 169e196. Wallh€ außer, E., Hussein, M.A., Becker, T., 2012. Detection methods of fouling in heat exchangers in the food industry. Food control 27, 1e10. Wilson, D.I., 2005. Challenges in cleaning: recent developments and future prospects. Heat. Transf. Eng. 26, 51e59. Yun, S.-H., Matheson, K., 1990. Estimation of amylose content of starches after €rke 42, 302e305. precipitation of amylopectin by concanavalin-A. Starch/Sta Zhao, Q., Wang, S., Müller-Steinhagen, H., 2004. Tailored surface free energy of membrane diffusers to minimize microbial adhesion. Appl. Surf. Sci. 230, 371e378. Zhao, Q., Liu, Y., 2006. Modification of stainless steel surfaces by electroless Ni-P and small amount of PTFE to minimize bacterial adhesion. J. Food Eng. 72, 266e272.
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Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Comparative cleaning tests with modified protein and starch residues C. Otto a, S. Zahn a, M. Hauschild a, F. Babick b, H. Rohm a, * a b
€t Dresden, 01062, Dresden, Germany Chair of Food Engineering, Technische Universita €t Dresden, 01062, Dresden, Germany Institute of Process Engineering and Environmental Technology, Technische Universita
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 October 2015 Received in revised form 7 January 2016 Accepted 18 January 2016 Available online 21 January 2016
The cleanability of food contact surfaces is determined by surface characteristics and the properties of the soiling material. Effects of selected physicochemical parameters of biopolymer soils on the removal from stainless steel were studied using a laboratory flow cell system with continuous removal measurement and, additionally, gravimetric detection. The results show the potential for improving cleaning processes on the basis of structural properties of the biopolymers. A higher cleaning efficiency was achieved when surface energy and cationic charge of the polymers was lower, indicating that adhesive interactions (van der Waals forces of attraction and electrostatic forces) between surface and polymer play a dominant role. The effects of pre-heating treatments of the soils, inducing molecular changes, on removal behavior were however not significant. The results indicate that a holistic view of product and process design is useful to develop efficient cleaning protocols. © 2016 Published by Elsevier Ltd.
Keywords: Cleaning Fouling Flow cell Stainless steel Protein Starch
1. Introduction Cleaning is essential to ensure hygienic food production and process performance. To improve efficiency of the cleaning of either heated or unheated surfaces, the knowledge of the specific soil properties that affect the performance of cleaning processes is essential. The understanding how organic soils respond to cleaning assists to finding decisions concerning cleaning fluid and method (Fryer and Asteriadou, 2009; Wilson, 2005). To execute efficient cleaning, adhesive and cohesive bindings of a soil onto contact surfaces must be overcome with minimal energy. The mechanisms of particle or polymer removal are influenced by the physicochemical properties of the soiling material and can be explained by thermodynamic and electrostatic fundamentals (Fourche, 1995; Michalski et al., 1997; Visser, 1995). According to the DLVO theory, total interaction energy comprises Lifshitzevan der Waals attractive forces, and attractive or repulsive electrostatic forces. Van der Waals interactions itself depend mainly on the Lifshitzevan der Waals (LW) surface free energy of the soil, and the electrostatic interaction is controlled by its electrokinetic potential. In contrast to electrostatic forces, van der Waals contributions are
€t * Corresponding author. Chair of Food Engineering, Technische Universita Dresden, Bergstrasse 120, 01062 Dresden, Germany. E-mail address: [email protected] (H. Rohm). http://dx.doi.org/10.1016/j.jfoodeng.2016.01.015 0260-8774/© 2016 Published by Elsevier Ltd.
insensitive to pH variation and electrolyte concentration in aqueous systems (Al-Amoudi and Lovitt, 2007; Zhao and Liu, 2006). Contact area effects, depending on soil particle roughness and morphology, play an additional role in polymer/surface interactions (Bobe et al., 2007; Liu et al., 2006). Several physical and chemical processes that alter the structure of organic soils may influence adhesion to contact surfaces. For example, acidification leads to systems with a higher cationic charge and hydrophobicity, which increases attractive forces so that removal becomes more difficult (Lopez et al., 2010; Stanga, 2010). Mauermann et al. (2009) reported a decrease of cohesive interactions within the soil and between soil and surface when surface and protein or starch-based soils have the same electrical charge. Itoh et al. (1995) explained electrostatic interactions between b-lactoglobulin and stainless steel by modified amino or carboxyl groups. For several starch types, Prochaska et al. (2007) showed that polar carboxyl group substitution resulted in enhanced surface activity and was accompanied by a viscosity decrease, especially at pH < 4.0. Drying removes water from a substrate and leads to changes such as volume reduction, solids redistribution and polymerization. Closer packing increases cohesive forces in films and makes removing and dispersing soils more difficult (Jonhed et al., 2008; Stanga, 2010). Additionally, LW surface free energy gLW of the soil will change. In context with tomato paste removal from stainless steel, Liu et al. (2006) reported on an adhesive strength of gLW ¼ 29.2 mJ/m2 for the unbaked system, and of
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gLW ¼ 30.6 mJ/m2 for tomato paste baked onto the steel. Detry et al. (2011) revealed an influence of the drying rate on starch granule adherence, which was attributed to physico-chemical wetting mechanisms. Exposure to more than 60 C also causes structural changes. It was reported that, when globular proteins undergo shape changes through denaturation, the exposure of hydrophobic sites leads to insolubilization. Surface hydrophobicity affects shape and compactness of adhering aggregates at soiling and their interfacial behavior during cleaning (Sava et al., 2005; Rabe et al., 2011). Goode et al. (2013) observed a significant temperatureinduced increase in adhesion forces between whey protein and stainless steel. When starch is gelatinized in water, colloidal solutions of high viscosity are obtained, in which the internal structures of the granules undergo considerable changes (Ratnayake and Jackson, 2006). The need to study and optimize cleaning has contributed to the development of specialized research techniques including flow cells, micromanipulation devices, or fluid dynamic gauging (Gordon et al., 2014). To evaluate soil adherence to surfaces, the residual soil mass after applying washing or cleaning protocols on solid surfaces can be determined (Handojo et al., 2009; Michalski et al., 1997). Flow cells were applied for comparative investigation of the cleaning behavior of different soils (Detry et al., 2009; Otto et al., 2014). Sensitive and rapid spectroscopic techniques may be used to continuously assess the amount of polymers removed from stainless steel, or in cleaning solutions (Boyd et al., 2001; Fickak et al., 2011). Several studies concerning the removal of several types of food soils were performed to investigate effects of surface modification treatments at different scale level, to determine properties of cleaning agents, or to develop or to monitor cleaning techniques (e.g., Boxler et al., 2013; Jeurnink and Brinkman, 1994; Saikhwan €ußer et al., 2012). The cleanet al., 2006; Otto et al., 2011; Wallha ing mechanism is also influenced by type and properties of the residues which are to be removed. A general relationship between soil characteristics and cleaning energy is given by Fryer and Asteriadou (2009) who developed the cleaning map to classify cleaning problems. However, little work has been done to extend the classification matrix to cohesive solid soil residues. This article demonstrates effects of selected modification procedures of starch and protein on the cleaning result. The parameters analyzed were the electrokinetic potential, surface free energy and conformational changes of a range of polymer residues treated by various process conditions such as acidification, drying and heating. 2. Materials and methods 2.1. Materials The test specimens had a surface roughness 0.8 mm, which is generally recommended for being used in the food industry (Gerhards and Schmid, 2013). The coupons from electropolished stainless steel 316L (40 20 1 mm3) were washed in 1% NaOH under sonication (15 min, 45 kHz, 160 W), rinsed in deionized water and dried on filter paper. Other surface properties of the steel coupons are: surface free energy, 38.3 mJ/m2; isoelectric point, 4e5 (Mauermann et al., 2009). Milei 80 whey protein (Milei GmbH, Leutkirchen, Germany), soy protein (Vegacon 90, Nutrition-Factory Alphacaps GmbH, Augustdorf, Germany), native waxy corn (WC) starch (Maisita 21.000, €rke GmbH, Aschach, Austria) and oxidized potato (OP) Agrana Sta starch (Aganadyn 20.050) were selected as model polymers. Moisture was determined by drying at 105 C to constancy (ISO, 1996), fat content was determined by the Soxhlet method (ISO, 1994), and protein by the Kjeldahl method (ISO, 2005) using
conversion factors of 6.25 for whey protein and 5.71 for soy protein. Ash was determined by incineration (ISO, 2007), and carbohydrate content was calculated as remaining difference to 100%. The amylose content of starch was analyzed using the concanavalin-A binding method (Yun and Matheson, 1990) with a commercial test kit (Megazyme Ltd., Bray, Ireland) (Table 1). 2.2. Soiling material preparation and modification Based on suggestions from literature (Boyd et al., 2001; Detry et al., 2011), each powder was suspended in deionized water at a concentration of 1% (w/v). After stirring for 12 h at room temperature, 100 mL liquid soil (thus containing 1 mg polymer) was pipetted onto a 2 cm2 area of the steel coupons and dried at 55 C for 1 h. In addition to this standard soiling procedure, modified polymer residues were prepared by changing pH and/or drying temperature, or by pre-heating the polymer suspensions to mimic processing conditions. pH adjustment was done with 0.5% HCl (pH 3), or with 0.5% NaOH (pH 9). To simulate thermal exposure, the soil films on the coupons were dried at 90 C for 1 h. Heat-induced modification of the polymers prior to soiling was achieved by heating the suspensions in a water bath to 82 C for 2 min. 2.3. Characterization of test soils 2.3.1. Electric properties Streaming potential and isoelectric point (IEP) of acidified or alkalized polymer suspensions were determined using a PCD 03 pH particle charge detector (Mütek, Herrsching, Germany; Mohammed et al., 2000). After filling 20 mL 1% (w/v) polymer suspension into the teflon cell, the streaming potential was induced by a plunger oscillating in the sample cell. Measurement temperature was 21 C. pH was adjusted by 0.1 mol/L HCL or 0.1 mol/L NaOH in 0.05 mL/min steps with an automatic titration device (Metrohm AG, 702 SM). The potentials (mV) were recorded in duplicate together with suspension pH by PCD-Titration software 1.6. In the titration curve the polymer IEP refers to pH at a streaming potential of zero. 2.3.2. Thermodynamic properties Contact angles of polymer films were determined by a modified Wilhelmy plate method using a digital DCAT11 tensiometer (Dataphysics, Filderstadt, Germany; Stiller et al., 2004). The stainless steel coupon was coated on both sides with thin coherent films by sequential pipetting of 100 mL polymer suspension on each side and subsequent drying at 55 C or 90 C for 1 h. The plate was attached to the instrument's microbalance (accuracy: 0.01 mg) and immersed into a-bromonaphthalene (a-BN) at 2 mm/min. The contact angle q ( ) was calculated from the wetting force F (mN) by cosq ¼ F/gL where g is the surface tension of a-BN (44.4 mJ/m2) and L (m) is plate perimeter. From the average contact angle cosine of the apolar a-BN, the Lifshitzevan der Waals component of the surface free energy gLW (mJ/m2) of dry soil films was calculated using gLW ¼ 11.1 (1 þ cosq)2 (Zhao et al., 2004). Analysis was done in 10 replicates. 2.3.3. Thermal properties Thermal properties of the biopolymer soil residues were analyzed in duplicate by differential scanning calorimetry (DSC) from 20 to 110 C at a heating rate of 10 K/min using a DSC Q200 with hermetic aluminum pans (TA Instruments, Eschborn, Germany). The soils were detached from coupons by placing them in 10 mL deionized water in a T480/H-2 ultrasonic bath (Elma GmbH, Singen, Germany) for 1 h and subsequently concentrated with an
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Table 1 Chemical composition of soil raw materials. Proteinsa
Parameter
Moisture (%) Protein (%) Fat (%) Total carbohydrates (%)b Amylose (%) Ash (%) a b
Starchesa
Whey
Soy
Waxy corn
Oxidized potato
8.3 ± 0.2 76.1 ± 0.1 5.2 ± 0.2 6.2 e 4.2 ± 0.2
7.1 ± 0.1 88.2 ± 0.3 0.5 ± 0.4 0.4 e 3.8 ± 0.1
11.7 ± 0.3 0.2 ± 0.1 0.3 ± 0.1 87.0 3.3 ± 0.2 0.8 ± 0.1
14.3 ± 0.2 0.3 ± 0.1 0.4 ± 0.1 84.1 29.7 ± 0.5 0.9 ± 0.1
Arithmetic mean ± standard deviation from (n ¼ 3) independent replicates. Total carbohydrates is [100 (moisture þ protein þ fat þ ash)].
Alpha 1e2 freeze dryer (Martin Christ Gefriertrocknungsanlagen GmbH, Osterode, Germany). 2.5 mg duplicates of the lyophilized samples were weighed into the pans, and 10 mL of deionized water was added. An empty pan was used as reference. Data analysis was done with Universal Analysis 2000 V4.4 software. 2.4. Cleaning experiments Cleaning experiments were carried out using a continuous flow cell system in the close circuit measuring arrangement (Otto et al., 2014) in four replicate measurements. Fig. 1 shows the main components of this device: a 50 mL reservoir with a magnetic stirrer for the cleaning fluid, the flow cell with a volume of 3 mL (width 6 cm, length 10 cm), and a peristaltic pump. After fixing the soiled stainless steel coupon in the flow cell and closing by four wing nuts, cleaning tests were performed with water (for protein removal) or Lugol's solution (0.007% J2, 0.014% KJ, pH 6.1; for starch removal) as cleaning liquids at a flow rate of 54 mL/min for 2 min periods. This flow rate corresponds to a Reynolds number of approx. 40 (Detry et al., 2009; Otto et al., 2014). To simulate prerinsing which is normally included in every cleaning procedure, cleaning liquid temperature was 21 C. To monitor cleaning progress, polymer concentration in the cleaning liquid was continuously measured using a Lambda 2S UV/ VIS spectrophotometer (see Fig. 1; Perkin Elmer, Rodgau, Germany). The concentration was directly proportional to absorbance up to 300 mg/mL protein in aqueous solution at 230 nm, and that of starch in Lugol's solution at 600 nm (Otto et al., 2014). Actual soil removal was calculated from the cleaning system fluid volume multiplied by protein or starch concentration in the fluid, and cleaning efficiency CE is further expressed as the ratio of soil removed at time t to the initial soil on the coupon (i.e., 1 mg). Based on the results of a previously published study (Otto et al., 2014), the removal after 2 min of continuous cleaning was defined as final cleaning efficiency (CEt¼2 min). In addition to spectroscopic determination, CEt¼2 min was
Fig. 1. Schematic flow cell device for cleaning tests.
gravimetrically determined for selected samples using the microbalance of the DCAT 11 tensiometer. After cleaning of the soiled steel coupons in the flow cell for 2 min, the coupons were dried for 1 h at 55 C and weighed with the built-in balance. Gravimetric cleaning efficiency GCEt¼2min was calculated by GCEt¼2min ¼ 100 [1 (mr mu)/(mi mu)] where mr, mi and mu are the mass of the remaining dried soil with coupon after cleaning, the initial mass of dried soil before cleaning, and the unsoiled dry coupon, respectively. 3. Results and discussion 3.1. Effect of acid or alkaline treatment Fig. 2 presents the streaming potential of the polymers at 21 C, which generally increased from pH 9 to pH 3. Whey protein and soy protein showed IEPs of approx. 4.6 and 5.1, respectively, which is in agreement with literature (Itoh et al., 1995). Mohammed et al. (2000) also demonstrated that a much higher temperature treatment intensity than applied in this study during soil drying is necessary to affect the IEP of dairy proteins to a significant extent. At acidic conditions (pH ~3), both starches exhibited a streaming potential close to zero. For OP starch, the potential remained negative over the investigated pH range because of the negative charges of carboxyl groups induced by oxidation (Kuakpetoon and Wang, 2008; Piorkowski and McClements, 2014). WC starch showed an IEP of ~5.5, indicating the contribution of the charged groups on granules surface. Fig. 3 shows removal profiles (i.e., cleaning characteristics measured by the continuous spectroscopic method) for whey
Fig. 2. pH dependency of the streaming potential of polymer soil solutions. Circles, whey protein; squares, soy protein; triangles, oxidized potato starch; diamonds, waxy corn starch. Data are arithmetic mean ± half deviation range from duplicate measurements.
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Fig. 3. pH dependency of the continuous spectroscopic cleaning profiles of whey soil solutions dried at 55 C. Data are arithmetic mean ± standard deviation from (n ¼ 4) replicate measurements. Gravimetric cleaning efficiency after 2 min is given for comparison.
suspensions of different pH, which were dried onto the steel coupons at 55 C. These profiles show that the cleaning efficiency calculated from the concentration in the soiling solution approaches a plateau and, after 2 min, gives a characteristic value to compare the effects of modification treatments. Our previous study (Otto et al., 2014) revealed that increasing the flow rate to 74 mL/ min (Re ~ 56) increased the initial cleaning rate but not the final CEt¼2min value. The whey soil pH shows a pronounced effect on the cleaning efficiency. Furthermore, CEt¼2min detected by the spectroscopic method is in good agreement with the gravimetric reference value GCEt¼2min. The determination of all other CEt¼2min profiles further referred to in this study was based on the recording of the respective cleaning profiles (data not shown). The appearance of these profiles was similar, meaning that soil removal started with a high rate and then approached the CEt¼2min value asymptotically. For a total of eight soils, the efficiency of cleaning for a 2 min time span was measured by both methods (Table 2). In only one of the comparisons (two-sample two-tailed t-test), a significant (P < 0.05) difference between the results was observed. The obtained values are highly correlated (R ¼ 0.98), and the linear regression function CEt¼2min (%) ¼ 0.23% þ 1.01 GCEt¼2min (%) clearly proves the reliability of the spectroscopic method. In addition, the average coefficients of variation of 11.3% (gravimetric) and 4.8% (spectroscopic determination) demonstrate the accuracy of the continuous flow cell method. Fig. 4 presents CEt¼2 min for all polymers at pH 3, 6 and 9 as a function of their streaming potential; this CEt¼2 min shows a general
Fig. 4. Effects of streaming potential and pH on the cleaning efficiency CEt¼2min of polymer suspensions dried at 55 . Circles, whey protein; squares, soy protein; triangles, oxidized potato starch; diamonds, waxy corn starch. Cleaning data are arithmetic mean ± standard deviation from (n ¼ 4) replicate measurements.
trend towards higher values when soil solution pH was higher. Because the IEP of stainless steel is reported as being in the acidic pH region (Mauermann et al., 2009), both the starches and the steel are negatively charged at alkaline pH. This suggests that electrostatic repulsive forces dominate and bonding forces are low. In the acidic milieu, a reduced charge of the starch particles decreases repulsive forces and causes lower cleaning efficiency. The proteins also exhibit a higher cleaning efficiency at higher pH; the highest removal was however observed when the soil solution was neutral (pH 6). Because repulsive forces diminish at this pH (which also implies lower adhesive removal) it is suggested that a cohesive removal behavior or other types of interaction forces (e.g., acidebase interactions) dominate. Generally, the protein film formation in the acidic range can be regarded as a very complex process. Gennadios et al. (1993) reported that film formation was inhibited by a poor protein dispersion around the IEP region whereas, at a pH away from the IEP, proteins may denature, unfold, or solubilize, thus exposing sulfhydryl and hydrophobic groups (Castner and Ratner, 2002). Such groups associate upon drying to form disulfide and hydrophobic bonding forces (Gennadios et al., 1993). It is common for all polymers that a higher electrokinetic potential in the acidic range enhances polymer removal and leads to a decrease of the strength of repulsive electrostatic forces between polymer and surface. Takahashi and Fukuzaki (2008) and Mauermann et al. (2012) also suggested that a better cleanability of metal oxide surfaces was probably caused by the small binding strengths of biopolymers onto negatively charged metal surfaces,
Table 2 Comparison of cleaning efficiency determination. Sample
Cleaning efficiency after 2 min (%)a Spectroscopic determination
Whey protein pH 9 Whey protein pH 6 Whey protein pH 3 Soy protein pH 6 Oxidized potato starch pH 9 Oxidized potato starch pH 6 Oxidized potato starch pH 3 Waxy corn starch pH 6
a
78.9 99.7a 66.0a 98.8a 52.8a 38.9a 26.0a 4.1a
± ± ± ± ± ± ± ±
2.40 3.29 0.91 2.91 3.75 3.88 1.49 1.47
Gravimetric determination 86.4b 98.1a 62.8a 94.0a 48.7a 42.2a 23.8a 4.5a
± ± ± ± ± ± ± ±
4.30 1.93 3.88 2.88 4.97 5.41 4.72 1.43
a Arithmetic mean ± standard deviation from (n ¼ 4) independent replicates. Mean values in a row with different superscripts differ significantly (P < 0.05; two-sample two-tailed t-test).
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and increases OP starch removal, with no apparent correlation to polymer LW surface free energy (within polymer type). However, concerning the entire polymer data set it appears that higher gLW and thus higher adhesion forces contribute to a higher adherence of polymer residues (the linear correlation coefficient is R ¼ 0.88), and their removal becomes more difficult. Differences in the relation between surface free energy and protein removal may result from the cohesive removal behavior or specific forces (electrostatic, acidebase) that counteract attractive van der Waals forces (Mauermann et al., 2012). Jonhed et al. (2008) reported that starch films formed at 95 C for 30 min are flexible, and their cohesive and adhesive strength decrease with increasing moisture content. Bobe et al. (2007) observed a similar trend between cleaning efficiency and surface energy by using one particular soil system and by varying the soiled surface. Additionally surface forces affect the polymer solubility (Van Oss, 1993), and it was also found that an intensive heat-induced modification treatment reduces protein solubility and increases starch solubility (Sava et al., 2005).
Fig. 5. Effect of Lifshitzevan der Waals (LW) surface free energy of the polymer on cleaning efficiency CEt¼2min. Circles, whey protein; squares, soy protein; triangles, oxidized potato starch; diamonds, waxy corn starch. Data are from cleaning of pH 6 soils and given as arithmetic mean ± standard deviation from (n ¼ 4) replicate measurements.
3.3. Effect of preheating treatment Table 3 presents the thermal DSC transition parameters and cleaning efficiency at 2 min of soil residues made from different preheated soil solutions. The pH of the solutions was only marginally affected by the heating step. For whey protein, OP and WC starch it is evident that the preheating treatment decreases the transition enthalpy but does not influence cleaning efficiency (P > 0.05). The ratio of the transition enthalpies of untreated to preheated samples reflects the degree of structural modifications that is caused by heat-induced denaturation or gelatinization. No transition peak and no unfolding temperature were detected in the soy protein thermograms of unheated and preheated residues, indicating a severe thermal treatment during the manufacture of the powder. The transition temperatures for whey protein, OP and WC starch reflect their conformational stability and are in agreement with literature (Ratnayake et al., 2009; Santos et al., 2006). It is especially the gelatinization temperature that can be critical for a specific cleaning action because intragranular binding forces of starches that are weakened facilitate the solubilization of granule constituents (Lopez et al., 2010). Our cleaning results indicate that the conformational changes of the polymers on the solid surface largely depend on polymer conformation in suspension and give only a minor contribution to their cleaning behavior. Furthermore, other cleaning studies in three-component systems with polymer, cleaning fluid and contacting stainless steel surface with a defined roughness from 0.2 to 1.8 mm reported no significant effects for food polymers (Gerhards and Schmid, 2013; Mauermann et al., 2012; Saikhwan et al., 2006).
and due because of their negative charge under alkaline conditions. €rnstro € m (2003) reported that the binding power of Jonhed and Ja cationic starch is higher than that of native starch because the ionic interactions are stronger than simple hydrogen bonds. In a fundamental study of solid particle detachment from membrane surfaces, Elzo et al. (1996) also observed that decreasing the pH in acid range increased the adhesive force between solid particles. 3.2. Effect of drying treatment Fig. 5 shows the effect of the temperature of drying soil solutions of pH 6 on LW energy and cleaning efficiency. The temperature increase from 55 to 90 C significantly (two-sample two-tailed ttest; P < 0.05) increases LW surface free energy of the globular proteins because of the heat-induced exposure of hydrophobic sites (Sava et al., 2005). LW surface free energy of starches was not affected, indicating a similar hydrophobic character of the materials after drying (Jonhed et al., 2008; Liu et al., 2006). The largest differences in surface free energy were observed between polymer types. For example, WC starch showed the highest gLW (approx. 40 mJ/m2). The energy values are in agreement with literature and allow a satisfactory comparison of the polymer surface properties (Cyras et al., 2008; Liu et al., 2006; Zhao and Liu, 2006). For example, Lawton (1995) used Wu's approach (Mauermann et al., 2009) to calculate the dispersive, non-polar surface energy of different extruded corn starch films. This measure with energy values in the range from 35 to 42 mJ/m2 roughly corresponds to the non-polar LW component of surface free energy that was measured in this study. A higher drying temperature of 90 C decreases protein removal
4. Conclusion The flow cell system was successfully used for the comparative
Table 3 Effect of soil solution preheating on thermal properties and cleaning efficiency CEt¼2min values of polymer residues. Polymer source
Unheated, 21 C pH
Whey protein Soy protein OP starch WC starch a
6.4 6.4 6.5 5.9
Pre-heated, 82 C, 2 min
Peak temperature Tp ( C) 76.1 ± 0.1 n.d. 63.6 ± 0.2 73.3 ± 0.3
a
1 a
Enthalpy DH (J g 6.9 ± 0.1 0 13.3 ± 0.1 12.9 ± 0.2
)
CEt¼2min (%) a
99.7 98.8a 38.9a 4.1a
± ± ± ±
3.2 2.9 3.8 1.4
b
pH
Peak temperature Tp ( C)
6.3 6.3 6.6 6.1
n.d. n.d. 64.2 ± 0.2 n.d.
a
Enthalpy DH (J g1) 0 0 1.2 ± 0.1 0
a
CEt¼2min (%)b 98.7a 99.8a 44.4a 6.2a
± ± ± ±
1.1 4.9 2.5 1.5
Arithmetic mean ± half deviation range from duplicate measurements. n.d., not detectable. Arithmetic mean ± standard deviation from (n ¼ 4) measurements. Mean values in a row with different superscripts differ significantly (P < 0.05; two-sample two-tailed ttest). b
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